1. Field of the Invention
The present invention relates to a structure of a camshaft bearing which supports a camshaft of an internal combustion engine.
2. Description of the Prior Art
In an internal combustion engine, an intake valve or an exhaust valve is opened or shut by a cam which rotates together with a camshaft. The camshaft is usually rotatably supported by a bearing including a portion of the cylinder head and a portion of the cam cap and its rotation is linked to the rotation of a crankshaft of the engine. The camshaft bearing is lubricated with oil which is supplied through the typically hollow camshaft. An orifice extends from the camshaft cavity to the surface of the camshaft under the bearing. Oil, pumped up from an oil pan, is fed to the bearing via the hollow camshaft and the orifice.
However, when the oil become hot and therefore has low viscosity, the oil is particularly thin and flows easily at the camshaft bearing, and thereby the thickness of the oil film becomes nearly zero at the contact portion of the surface of the camshaft with the inner surface of the camshaft bearing. Moreover, when the engine is running at very low speeds (e.g., less than 100 rpm), the oil pump which is driven by the crankshaft also rotates slowly, and thereby the oil pressure from the oil pump is very low. Therefore, when the engine is running at very low speeds with the oil hot and therefore having low viscosity, the supply of oil to the camshaft bearing is often insufficient. When the lubrication of the camshaft bearing is not satisfactory, the camshaft is liable to momentarily bind at the camshaft bearing and generate a noise. This momentary binding is liable to happen at a position where a valve spring applies a force on the camshaft bearing when the cam is pushed by a valve spring in the working direction of the valve spring force.
As an example of an improved bearing structure in which a shaft is urged in a direction normal to the axis of the shaft for the purpose of holding an appropriate volume of oil and obtaining sufficient lubrication of a bearing, Japanese Utility Model Publication SHO No. 57-141228 discloses an improved structure for the small end of a connecting rod, but there is no example of a camshaft bearing having been structurally improved for the purpose of obtaining sufficient lubrication even under severe engine condition.